The present invention relates to a method for press-hardening a metallic formed structure made of a hardenable steel sheet, and to a device for carrying out the method.
The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention.
German Pat. No. DE 102 54 695 B3 describes a production of a metallic formed structure, in particular a chassis structure, from an unhardened, hot-formable steel sheet by initially shaping a semi-finished product through cold-forming, in particular through deep-drawing, to a structure blank, which is subsequently cut at the edges to an edge contour that approximates the contour of the structure. This cut blank is subsequently press-hardened in a hot-forming tool. This process suffers shortcomings, when high-strength press-hardened steels are involved which typically do not permit deep-drawling of complex structures without a hold-down device or sheet holder so as to prevent the formation of wrinkles when the sheet metal is drawn into the female die. The sheet holders can also be provided with a cooling device. However, the edge regions of the steel sheet are then also hardened, resulting in increased wear of the cutting tools used to subsequently cut the press-hardened structures to size. It would be desirous to provide the press-hardened structure with an edge region where the press-hardened structure is softer or has a smaller hardness. The pressed structure could then be cut in this region without causing increased tool wear of the cutting tools. However, such partially soft regions cannot be formed with conventional sheet holders, because those regions which are drawn inwardly into the female die during the forming process make prior contact with the cooled sheet holders and are therefore hardened. Very high-strength materials can then attain a hardness greater than 1,500 MPa in the marginal regions. Such high-strength materials can be cut to size only in a very complex manner.
It would therefore be desirable and advantageous to address prior art shortcomings and to provide an improved method and device for press-hardening a metallic formed structure which is suitable for implementing partially unhardened regions also in the marginal region of the press-hardened steel sheet, even if these regions are arranged in the region of cooled sheet holders at the beginning of the forming process.